Polyethylene naphthalate (PEN) is a naphthalate-based polyester that is used in the production of fiber, film and rigid packaging applications. Like most thermoplastic polyesters, PEN is also produced by melt phase polymerization, which gives the amorphous polymer a medium intrinsic viscosity (I.V.). To increase the I.V., amorphous PEN chips are crystallized and then subjected to solid state polymerization (SSP). The pre-crystallization process used for the related polyester polyethylene terephthalate (PET) cannot be used for PEN because of the differences in their physical and chemical properties. For example, PEN's Tg is about 120° C., its melting point, Tm, is about 270° C., and its softening point or sticking temperature is in the range of about 140° C. to about 150° C. Hence, the crystallization temperature used for PEN is normally in the range of about 180° C. to about 220° C., which is much higher than that used for PET, wherein PEN chips undergo a sticky stage when they cross the softening point or sticking temperature. To avoid sticking or lumping of PEN chips, agitated vessels or fluidized beds are used.
Several methods have been adopted to precrystallize amorphous PEN before it is subjected to SSP.
U.S. Pat. No. 4,798,883 describes a modified PEN process with enhanced crystallization characteristics of amorphous PEN by the addition of small amount of polyether glycols such as polytetramethylene glycol and polyethylene glycol.
U.S. Pat. No. 4,963,644 describes precrystallization of amorphous PEN prepolymer which comprises: (1) heating the amorphous PEN prepolymer to a temperature that is within the range of about 80° C. to about 140° C. in the presence of a stream of an inert gas or under a vacuum to devolatilize the amorphous PEN prepolymer; and (2) subsequently heating the devolatilized PEN prepolymer to a temperature within the range of about 150° C. to about 260° C. while providing agitation to produce crystallized PEN prepolymer. The precrystallized PEN prepolymer is subsequently subjected to SSP by heating it to a temperature of about 50° C. to about 1° C. below its sticking temperature for a period of time sufficient to produce the high molecular weight PEN resin.
U.S. Pat. No. 5,412,063 describes the manufacture of PET and PEN chips wherein the amorphous strands have a hole in the middle. The chips do not need any special precrystallization procedure, such as devolatilization, prior to SSP.
U.S. Pat. No. 5,523,361 describes a precrystallization procedure for PEN involving coating the amorphous PEN chips with ethylene carbonate, which avoids the devolatilization procedure before SSP.
U.S. Pat. Nos. 5,744,578 and 5,750,644 describe processes for the crystallization of a PEN prepolymer containing volatile components. The temperature is maintained in the range of about 20° C. above the Tg and about 10° C. below the Tm while the PEN prepolymer is agitated. The PEN prepolymer is present as chips or pellets. The crystallization is performed in the presence of a liquid or a mixture of liquids that do not degrade the polymer during crystallization. The liquid or liquids employed may be those whose vapor pressure at the amorphous PEN softening temperature is insufficient to forestall expansion of the pellets. When this class of liquid or liquids is employed, the crystallization vessel is pressurized with an inert gas, including air, to prevent prepolymer expansion. If the liquid or liquids possess sufficiently high vapor pressures at the PEN softening temperature, the pellets do not expand even without the inert air pressurization, thereby avoiding deformation of the PEN chips during crystallization.
U.S. Pat. No. 6,124,423 describes a PEN copolymer process wherein the devolatilization prior to SSP can be avoided by modifying the PEN with diethylene glycol (DEG). Crystallizing the PEN copolymer solids by heating the solids to at least their sticking temperature at an average rate of at least 10° C./minute, to form agglomerate-free crystallized solids. The process may be performed under agitation. The pellets heated up during the crystallization step do not agglomerate or suddenly expand as the temperature rises to and through the sticking temperature of the PEN copolymers. The process allows one to rapidly heat up PEN during crystallization, and do so without conducting a prior drying step or devolatilization step.
All the above precrystallization procedures prior to SSP are cumbersome and involve invariably more than one stage of precrystallization, thereby risking deterioration in quality.
A need therefore exists for a simpler crystallization procedure that uses only a single stage of crystallization.